1. Field of the Invention
This invention relates to apparatus for vacuum pickup of an article. More particularly, this invention relates to an apparatus having vacuum gripper means and retractable spacer means which is capable of picking up a single article from a variable height stack of articles of various thicknesses, weights, materials, and porosity.
2. Description of the Related Art
In the automated handling of articles, particularly flat articles such as pieces of mail or stacks of paper, it is necessary to be able to pick up a single article or sheet from a stack of articles or sheets of various thicknesses and weights. The use of vacuum means to affect such single transport has created problems in the past because the use of a sufficient vacuum to enable a heavy article to be lifted, e.g., a 2-3 lb article, can also result in multiple pickup of light weight articles such as cards, e.g., postcards, or lightweight airmail, due to bleed through of the vacuum through the thin and/or porous article.
Conversely, reducing the amount of vacuum used, to avoid multiple pickup of light weight and/or porous articles, results in a decrease of the ability of the apparatus to carry heavier articles, particularly those having a low coefficient of friction with the vacuum means in contact with the article.
Other attempts to overcome such multiple pickup, including experimentation with mechanical means to bend light weight highly porous articles to induce airflow between the light weight porous articles, have also not been successful.
In the prior art, avoiding double feeding of light weight articles of similar size, thickness, weight, porosity etc., such as sheets of paper, has been attempted using mechanical means to kink, curve, or in other ways distort the top sheet to avoid double sheet feed. For example, Kerscher U.S. Pat. No. 765,738 teaches a vacuum mechanism for pickup of articles which uses a sucker and a member which vertically spaces the sucker from the top of the stack of articles to prevent pickup of more than one article by causing a kink in the article. The vertical spacing of the member from the stack of articles and the horizontal spacing of the member from the sucker is preset for the weight of the articles in the particular stack of articles being picked up. If the articles are heavier, the member is raised and moved farther from the sucker. If the articles are light, the member is moved down and closer to the sucker to produce a more pronounced kink in the article being raised to prevent multiple feeding.
Rives U.S. Pat. No. 3,008,748 describes a suction pickup head which comprises a central tube with a slidable sleeve mounted around the tube and an end on the slidable sleeve which is angled with respect to the central tube. When the pickup head is lowered onto a stack of sheets, the uppermost sheet is sucked up, causing the sleeve to slide back along the tube. During the upward movement of the sheet, a portion of the sheet is held against the end of the tube and a portion is held against the bottom face of the sleeve which, being at an angle to the tube, causes buckling of the sheet to promote separation of the sheet from the other sheets.
Glaser et al U.S. Pat. No. 3,773,318 discloses a sheet feeder and holder assembly wherein a pickup assembly contains sucker cylinders and fingers which both engage the top sheet of a stack. The sucker cylinder comprises a tube with a slidable cylinder around the tube which is spring biased downward toward the stack. When the sucker cylinders suck up the top sheet, the suction force retracts the slidable cylinders. However, the fingers remain in a stationary position causing the sheet to bend or flex about a line extending between the fingers which decreases the likelihood that the next sheet in the stack will also be lifted with the top sheet.
Sato et al U.S. Pat. No. 4,428,793 describes a mechanism for removal of a coverslip from a stack of same using a supporting body comprising an adhering cup and a pushing body which is spring biased to downwardly protrude beyond the end of the adhering cup. When the supporting body is lowered over a stack of coverslips, the pushing body first contacts the stack and is forced upward by the force of the descending supporting body until the adhering cup also contacts the stack. When the adhering cup makes contact with the stack, a vacuum is turned on within the adhering cup and the supporting mechanism is raised with the uppermost coverslip being lifted by the vacuum in the adhering cup. The spring loaded pushing body now pushes downward against this sheet causing it to bend. This bending causes a sliding action against a second coverslip which may be clinging to the top one so that only the top coverslip will be taken.
Hancock U.S. Pat. No. 4,589,648 teaches a pinch-action suction cup which contains a protrusion within one side of the vacuum bellows. This protrusion creates a pressure point which is asymmetric of the vacuum force center causing a top sheet to bend sharply around the protrusion, causing the top sheet to slide over a second sheet which permits the intrusion of an air boundary between the top and second sheet and opens a void volume between the two sheets under the suction head. This sliding and bending of the top sheet, relative to the second sheet, is said to prevent double feeding.
However, despite the fact that the prior art claims to have achieved success in avoiding double feeding of light weight articles of similar or identical thickness, weight, porosity, etc., there remains a problem in attempting to affect single item pickup of a random assortment of articles of varying thickness, weight, porosity, etc., as well as varying height of the stack of articles. For example, even when the use of spacers or the like to cause distortion in a top sheet are successful, as alleged by the prior art, in preventing multiple feeding of the same type of sheet or article, the mechanism necessary to perform this bending action can interfere with the transport of rigid or heavy articles.
Therefore, there remains a need for a vacuum apparatus capable of picking up single articles from a random assortment or stack of articles, particularly flat articles, of varying thicknesses, porosity, and weight, and where even the height of the stack is variable.